Abstract

Seven cores containing anoxic sediments recovered in the Bannock Basin were analysed in their chemical, physical and biological characters. Bannock Basin is a large deep subcircular depression near the deformation front of the Mediterranean Ridge facing the Sirte Abyssal Plain. The basin is divided into several sub-basins aligned along a rim-syncline surrounding a central domelike area. High-density brines, deriving from submarine dissolution of Messinian evaporites, are present under — 3200 m: a distinct interface separating normal sea water from the brines coincides with a change from normal pelagic sedimentation to anoxic sediments. Three facies: a homogeneous one typical of a Holocenic pelagic turbidite dated 3500 yr B.P.; a laminated facies, characterized by thin laminae which differ in colour and/or grain size and a non-laminated one containing small turbidites and/or debris flows are identified. All sediments are younger than 70.000 yr B.P., non-bioturbated, dark in colour, rich in H2S and contain large gypsum crystals. The organic carbon content is higher than in normal Eastern Mediterranean pelagic sediments, but lower than in true sapropels, and its variation opposes the CaCO3 one. The laminated facies has been investigated carefully in order to find a relation between colour and chemical, physical, mineralogical and biological characters. No substantial change has been recorded in the dark vs. light laminae in terms of major and minor elements, of clay minerals, of carbonate content. Only the organic C content is definetly higher in the dark laminae. A microscopic study carried out by means of smear slides documented a difference between the light laminae, where calcitic bioclast prevails, and the dark laminae dominated by siliceous plankton. All these data suggest the laminated facies derives from the presence of the interface, which interrupts the normal pelagic fall out, and is unrelated to climatic cycles.

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